The invention relates to a tool for measuring a clamping force exerted by a movable spindle of a length measuring instrument.
Precision length measuring instruments, usually termed micrometers, consist essentially of a body, a spindle mounted in the body so that it projects from the body and is axially movable relative thereto, an anvil fixed to the body in line with the spindle, means for causing the spindle to move and generally including a nut having a knurled gripping surface, and a stress limiter interposed between the spindle and the moving means.
In the case of an external micrometer, the article whose length is to be measured is placed between the free end of the spindle and the anvil, and the moving means is operated to bring the spindle closer to the anvil until the article is clamped. The stress limiter interrupts the action of the moving means as soon as the stopping force is reached.
The precise knowledge of this force is of considerable importance, and French Standard Ell-095 specifies that in principle it should be between 5 and 20 newtons.
Moreover, the variation of this force throughout the measurement should not exceed .+-.2.5 newtons.
If the stopping force is too weak, the clamping of the article is no longer ensured and it becomes difficult to measure in satisfactory circumstances. On the other hand, if it is excessive, mechanical deformations are produced which result in measurement errors.
It is therefore necessary periodically to measure the clamping force exerted by the movable spindle to check that the instrument meets the required standard, but, to the inventor's knowledge, there is no simple tool for this purpose in existence.